Lubricant composition in particular for limiting friction

ABSTRACT

The present application relates to a lubricant composition comprising:
         at least one base oil;   from 0.05 to 1.5% by weight of an organomolybdenum compound; and   from 0.005 to 1% by weight of TiO 2  particles.

The present application relates to new lubricant compositions, in particular for an internal combustion engine, in particular for a vehicle engine, in particular for a motor vehicle. In a particularly preferred manner, the present invention relates to lubricant compositions for reducing friction between parts.

It is known to use molybdenum-based compounds as a friction modifier in lubricant compositions, in particular organomolybdenum compounds. These organomolybdenum compounds form MoS₂ through tribochemical decomposition, in the presence of iron, through contacts causing the friction, which makes it possible to limit the friction between the parts. However, during this tribochemical decomposition, a part of the organomolybdenum compounds is converted into molybdenum oxide and molybdenum oxysulphide which have no effect on the limitation of friction. It is therefore generally necessary to use large quantities of organomolybdenum compounds (to optimize the amount of MoS₂ produced) in the lubricant compositions but which can have adverse effects on the lubricated parts, in particular corrosion, fouling, and deterioration of the coatings in the case of parts coated with carbon.

In particular CN104450069 discloses lubricant compositions comprising MoS₂ and titanium particles, including TiO₂, to reduce friction and wear of parts.

In particular U.S. Pat. No. 5,709,936 also discloses coating of parts comprising titanium, in particular titanium carbide or nitride and/or molybdenum in order to reduce friction.

WO2011112372 also discloses lubricant compositions comprising organomolybdenum compounds and liposoluble compounds based on titanium.

There is interest in providing lubricant compositions that reduce the friction between parts and also reduce the wear of these parts.

There is also interest in providing lubricant compositions for reducing the amount of molybdenum compound introduced.

An object of the present application is to provide lubricant compositions for reducing friction between parts.

An object of the present application is also to provide such lubricant compositions to also reduce the wear of parts.

An object of the present application is also to provide such compositions to permit duration of anti-friction action over a longer period.

Another object of the present application is also to provide a lubricant composition reducing the content of a molybdenum compound.

Still other objects will become apparent upon reading the description of the invention which follows.

These objectives are fulfilled by the present invention which provides a lubricant composition comprising:

at least one base oil;

from 0.05 to 1.5% by weight of an organomolybdenum compound; and

from 0.005 to 1% by weight of TiO₂ particles.

In the context of the present invention, the term “organomolybdenum compound” is intended to mean compounds comprising molybdenum and an organic part, i.e. a part comprising carbon atoms. It must therefore be understood that molybdenum disulfide (MoS₂) is not an organomolybdenum compound according to the invention.

Preferably, the TiO₂ particles have an average size of between 10 nm and 1 μm, preferably between 10 nm and 500 nm, more preferably between 10 nm and 250 nm, and even more preferably between 10 nm and 150 nm. In the context of the present invention, mean particle size means the average particle diameter. This average diameter may be measured by any method known to those skilled in the art such as, for example, counting by optical or electronic microscopy, measurement by light scattering or by laser diffraction. The light scattering measurement may, in particular, be carried out using a Malvern Zetasizer (so-called “Dynamic Light Scattering” technique), the laser diffraction measurement may be made with a Malvern Mastersizer.

Particles of TiO₂ may come, in particular, from rutile or anatase.

Preferably, the composition according to the invention comprises from 0.08 to 1% by weight of organomolybdenum compound relative to the total weight of the lubricant composition.

Preferably, the composition according to the invention comprises from 0.01 to 0.8% by weight of TiO₂ particles relative to the total weight of the lubricant composition.

Advantageously, the addition to the lubricant composition of TiO₂ particles makes it possible to reduce the amount of organomolybdenum compound normally used to limit friction. In fact, without wishing to be bound by any theory, the inventors have surprisingly shown that the mixture of organomolybdenum compound and TiO₂ particles advantageously led to an excellent conversion rate of organomolybdenum compound MoS₂ while avoiding the generation of molybdenum oxysulfide in the lubricant composition and thus the loss of active molybdenum to prevent friction. This specific combination of organomolybdenum and titanium particles according to the invention therefore makes it possible to considerably reduce friction at a lower content of organomolybdenum compound.

By organomolybdenum compound according to the invention is meant any organomolybdenum compound that is soluble in an oil, especially in a base oil.

The organomolybdenum compound according to the present invention may be chosen from organic complexes of molybdenum comprising at least one molybdenum (Mo) chemical element and at least one ligand such as a carboxylate ligand, an ester ligand, an amide ligand, a dithiophosphate ligand, or a dithiocarbamate ligand.

For example, organic complexes of molybdenum with carboxylates, esters, amides may be obtained by reacting molybdenum oxide or ammonium molybdates with fatty substances, glycerides, fatty acids or fatty acid derivatives (esters, amines, amides, . . . ).

For the purpose of the invention, the carboxylate ligands, the ester ligands and the amide ligands are free of sulfur and phosphorus.

In one embodiment, the organomolybdenum compound of the invention is selected from molybdenum complexes with amide ligands, mainly prepared by reaction of a molybdenum source, which may be, for example, molybdenum trioxide, and a derivative of amine, and of fatty acids comprising, for example, from 4 to 36 carbon atoms, such as, for example, the fatty acids contained in vegetable or animal oils.

The synthesis of such compounds is described, for example, in patents U.S. Pat. No. 4,889,647, EP0546357, U.S. Pat. No. 5,412,130 or EP1770153.

In a preferred embodiment of the invention, the organomolybdenum compound is chosen from among organic complexes of molybdenum with amide ligands obtained by reaction:

-   -   (i) of a mono, di or tri glyceride fatty substance, or fatty         acid,     -   (ii) of an amine source of formula (A):

in which:

-   -   X¹ represents an oxygen atom or a nitrogen atom,     -   X² represents an oxygen atom or a nitrogen atom,     -   n or m represents 1 when X¹ or X² respectively represents an         oxygen atom,     -   n or m represents 2 when X¹ or X² respectively represents a         nitrogen atom,     -   (iii) and of a molybdenum source selected from molybdenum         trioxide or molybdates, preferably ammonium molybdate.

Advantageously, the molybdenum source is used in an amount sufficient to provide 0.1 to 30% of molybdenum relative to the total weight of the molybdenum organic complex with amide ligands, more preferably from 0.1 to 20.0% of molybdenum.

In one embodiment according to the invention, the organomolybdenum compound may comprise from 2 to 8.5% by weight of molybdenum relative to the total weight of the organic molybdenum complex with amide ligands.

Preferably, the organomolybdenum compound comprises at least one organic complex of molybdenum of formula (III) or (IV), alone or as a mixture:

in which:

-   -   X¹ represents an oxygen atom or a nitrogen atom;     -   X² represents an oxygen atom or a nitrogen atom;     -   n represents 1 when X¹ represents an oxygen atom and m         represents 1 when X² represents an oxygen atom;     -   n represents 2 when X¹ represents a nitrogen atom and m         represents 2 when X² represents a nitrogen atom;     -   R₁ represents a linear or branched, saturated or unsaturated         alkyl group comprising from 4 to 36 carbon atoms, preferably         from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon         atoms;

in which:

-   -   X¹ represents an oxygen atom or a nitrogen atom;     -   X² represents an oxygen atom or a nitrogen atom;     -   n represents 1 when X¹ represents an oxygen atom and m         represents 1 when X² represents an oxygen atom;     -   n represents 2 when X¹ represents a nitrogen atom and m         represents 2 when X² represents a nitrogen atom;     -   R₁ represents a linear or branched, saturated or unsaturated         alkyl group comprising from 4 to 36 carbon atoms, preferably         from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon         atoms;     -   R₂ represents a linear or branched, saturated or unsaturated         alkyl group comprising from 4 to 36 carbon atoms, preferably         from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon         atoms.

Advantageously, the organic complex of molybdenum of formula (III) or (IV) is prepared by reaction:

-   -   (i) of a mono, di or tri glyceride fatty substance, or fatty         acid,     -   (ii) of diethanolamine or 2-(2-aminoethyl) aminoethanol,     -   (iii) and of a molybdenum source selected from molybdenum         trioxide or molybdates, preferably ammonium molybdate.

More advantageously, the organic complex of molybdenum of formula (III) consists of at least one compound of formula (III-a) or (III-b), alone or as a mixture:

in which R₁ represents a linear or branched, saturated or unsaturated alkyl group comprising from 4 to 36 carbon atoms, preferably from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon atoms,

in which R₁ represents a linear or branched, saturated or unsaturated alkyl group comprising from 4 to 36 carbon atoms, preferably from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon atoms.

As examples of sulfur-free molybdenum complexes according to the invention, mention may be made of Molyvan 855® sold by the company Vanderbilt.

In another embodiment of the invention, the organomolybdenum compound is selected from organic complexes of molybdenum with dithiophosphate ligands or organic complexes of molybdenum with dithiocarbamate ligands.

Within the meaning of the invention, organic complexes of molybdenum with dithiophosphate ligands are also called molybdenum dithiophosphates or Mo-DTP compounds and organic complexes of molybdenum with dithiocarbamate ligands are also called molybdenum dithiocarbamates or Mo-DTC compounds.

In a more preferred embodiment of the invention, the organomolybdenum compound is selected from molybdenum dithiocarbamates.

Mo-DTC compounds are complexes formed by a metal core of molybdenum bound to one or more ligands, the ligand being an alkyl dithiocarbamate group. These compounds are well known to those skilled in the art.

In one embodiment of the invention, the Mo-DTC compound may comprise from 1 to 40%, preferably from 2 to 30%, more preferably from 3 to 28%, advantageously from 4 to 15% by weight of molybdenum, relative to the total weight of Mo-DTC compound.

In another embodiment of the invention, the Mo-DTC compound may comprise from 1 to 40%, preferably from 2 to 30%, more preferably from 3 to 28%, advantageously from 4 to 15% by weight of sulfur, based on the total weight of Mo-DTC compound.

In another embodiment of the invention, the Mo-DTC compound may be chosen from among those whose nucleus has two molybdenum atoms (also called dimeric Mo-DTC) and those whose nucleus has three molybdenum atoms (also called Trimeric Mo-DTP).

In another embodiment of the invention, the trimeric Mo-DTC compounds have the formula Mo₃S_(k)L_(n) in which:

-   -   k represents an integer at least equal to 4, preferably ranging         from 4 to 10, advantageously from 4 to 7,     -   n is an integer ranging from 1 to 4, and     -   L is an alkyl dithiocarbamate group comprising from 1 to 100         carbon atoms, preferably from 1 to 40 carbon atoms, preferably         from 3 to 20 carbon atoms.

As examples of trimeric Mo-DTC compounds according to the invention, mention may be made of the compounds and their methods of preparation as described in the documents WO 98/26030 and US 2003/022954.

In a preferred embodiment of the invention, the Mo-DTC compound is a dimeric Mo-DTC compound.

Examples of dimeric Mo-DTC compounds include compounds and methods for their preparation as described in EP 0757093, EP15 0719851, EP 0743354 or EP 1013749.

The dimeric Mo-DTC compounds generally correspond to the compounds of formula (V):

in which:

-   -   R₃, R₄, R₅, R₆, which may be identical or different,         independently represent a hydrocarbon group chosen from alkyl,         alkenyl, aryl, cycloalkyl or cycloalkenyl groups,     -   X₃, X₄, X₅, X₆, which may be identical or different,         independently represent an oxygen atom or a sulfur atom.

For the purposes of the invention, the term “alkyl group” means a linear or branched, saturated or unsaturated hydrocarbon-based group comprising from 1 to 24 carbon atoms, preferably from 4 to 18 carbon atoms.

In one embodiment according to the invention, the alkyl group is selected from among the group consisting of methyl, ethyl, propyl, isopropyl, n-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, isotridecyl, tetradecyl, hexadecyl, stearyl, icosyl, docosyl, tetracosyl, triacontyl, 2-ethylhexyl, 2-butyloctyl, 2-butyldecyl, 2-hexyloctyl, 2-hexyldecyl, 2-octyldecyl, 2-hexyldodecyl, 2-octyldodecyl, 2-decyltetradecyl, 2-dodecylhexadecyl, 2-hexadecyloctadecyl, 2-tetradecyloctadecyl, myristyl, palmityl and stearyl.

For the purposes of the present invention, the term “alkenyl group” means a linear or branched hydrocarbon group comprising at least one double bond and comprising from 2 to 24 carbon atoms. The alkenyl group may be chosen from among vinyl, allyl, propenyl, butenyl, isobutenyl, pentenyl, isopentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tetradecenyl and oleic.

Aryl group within the meaning of the present invention means a polycyclic aromatic hydrocarbon or an aromatic group, substituted or not with an alkyl group. The aryl group may comprise from 6 to 24 carbon atoms.

In one embodiment, the aryl group may be selected from the group consisting of phenyl, toluyl, xylyl, cumenyl, mesityl, benzyl, phenethyl, styryl, cinnamyl, benzhydryl, trityl, ethylphenyl, propylphenyl, butylphenyl, pentylphenyl, hexylphenyl, heptylphenyl, octylphenyl, nonylphenyl, decylphenyl, undecylphenyl, dodecylphenyl, phenylphenyl, benzylphenyl, phenylstyrene, p-cumylphenyl and naphthyl.

By cycloalkyl group within the meaning of the present invention is meant a polycyclic or cyclic hydrocarbon, substituted or not with an alkyl group.

By cycloalkenyl group within the meaning of the present invention is meant a polycyclic or cyclic hydrocarbon, substituted or not with an alkyl group, and comprising at least one unsaturation.

Cycloalkyl groups and cycloalkenyl groups may comprise from 3 to 24 carbon atoms.

For the purpose of the present invention, the cycloalkyl groups and the cycloalkenyl groups may be chosen, in a non-limiting manner, from the group consisting of cyclopentyl, cyclohexyl, cycloheptyl, methylcyclopentyl, methylcyclohexyl, methylcycloheptyl, cyclopentenyl, cyclohexenyl, cycloheptenyl, methylcyclopentenyl, methylcyclohexenyl.

In a preferred embodiment of the invention, R₃, R₄, R₅, R₆, which may be identical or different, independently represent an alkyl group comprising from 1 to 24 carbon atoms, preferably from 4 to 18 carbon atoms, or an alkenyl group comprising from 2 to 24 carbon atoms.

In one embodiment of the invention, X₃, X₄, X₅, X₆ may be the same and may be a sulfur atom.

In another embodiment of the invention, X₃, X₄, X₅, X₆ may be the same and may be an oxygen atom.

In another embodiment of the invention, X₃ and X₄ may represent a sulfur atom and X₅ and X₆ may represent an oxygen atom.

In another embodiment of the invention, X₃ and X₄ may represent an oxygen atom and X₅ and X₆ may represent a sulfur atom.

In another embodiment of the invention; the ratio of the number of sulfur atoms to the number of oxygen (S/O) atoms of the Mo-DTC compound may vary from (1/3) to (3/1).

In another embodiment of the invention, the Mo-DTC compound of formula (V) may be chosen from a symmetrical Mo-DTC compound, an asymmetrical Mo-DTC compound and their combination.

By a symmetrical Mo-DTC compound according to the invention is meant an Mo-DTC compound of formula (V) in which the groups R₃, R₄, R₅, R₆ are identical.

By an asymmetrical Mo-DTC compound according to the invention is meant a Mo-DTC compound of formula (V) in which the groups R₃ and R₄ are identical, the groups R₅ and R₆ are identical and the groups R₃ and R₄ are different from the groups R₅ and R₆.

In a preferred embodiment of the invention, the Mo-DTC compound is a mixture of at least one symmetrical Mo-DTC compound and at least one asymmetrical Mo-DTC compound.

In one embodiment of the invention, R₃ and R₄, which are identical, represent an alkyl group comprising from 5 to 15 carbon atoms, preferably from 8 to 13 carbon atoms, and R₅ and R₆, which are identical, represent an alkyl group comprising from 5 to 15 carbon atoms, preferably from 8 to 13 carbon atoms, and the groups R₃ and R₄ are identical or different from the groups R₅ and R₆.

In another preferred embodiment of the invention, R₃ and R₄, which are identical, represent an alkyl group comprising from 6 to 10 carbon atoms and R₅ and R₆, which are identical, represent an alkyl group comprising from 10 to 15 carbon atoms, and the groups R₃ and R₄ are different from the groups R₅ and R₆.

In another preferred embodiment of the invention, R₃ and R₄, which are identical, represent an alkyl group comprising from 10 to 15 carbon atoms and R₅ and R₆, which are identical, represent an alkyl group comprising from 6 to 10 carbon atoms, and the groups R₃ and R₄ are different from the groups R₅ and R₆.

In another preferred embodiment of the invention, R₃, R₄, R₅, R₆, which are identical, represent an alkyl group comprising from 5 to 15 carbon atoms, preferably from 8 to 13 carbon atoms.

Advantageously, the compound Mo-DTC is chosen from compounds of formula (V) in which:

-   -   X₃ and X₄ represent an oxygen atom,     -   X₅ and X₆ represent a sulfur atom,     -   R₃ represents an alkyl group comprising 8 carbon atoms or an         alkyl group comprising 13 carbon atoms,     -   R₄ represents an alkyl group comprising 8 carbon atoms or an         alkyl group comprising 13 carbon atoms,     -   R₅ represents an alkyl group comprising 8 carbon atoms or an         alkyl group comprising 13 carbon atoms,     -   R₆ represents an alkyl group comprising 8 carbon atoms or an         alkyl group comprising 13 carbon atoms.

Thus, advantageously, the Mo-DTC compound is chosen from among compounds of formula (V-a)

in which the groups R₃, R₄, R₅, R₆ are as defined for formula (V).

More advantageously, the compound Mo-DTC is a mixture:

-   -   of an Mo-DTC compound of formula (V-a) in which R₃, R₄, R₅, R₆         represent an alkyl group comprising 8 carbon atoms,     -   of an Mo-DTC compound of formula (V-a) in which R₃, R₄, R₅, R₆         represent an alkyl group comprising 13 carbon atoms, and/or     -   an Mo-DTC compound of formula (V-a) in which R₃ and R₄ represent         an alkyl group comprising 8 carbon atoms and R₅ and R₆ represent         an alkyl group comprising 13 carbon atoms.

Examples of Mo-DTC compounds that may be mentioned are Molyvan L®, Molyvan 807® or Molyvan 822® products marketed by R.T. Vanderbilt Company or Sakura-lube 200®, Sakura-lube 165® and Sakura-lube products. 525® or Sakura-lube 600® marketed by the company Adeka.

The lubricant composition according to the invention may further comprise at least one anti-wear additive chosen from phospho-sulfur-containing additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates.

In general, the lubricant composition according to the invention may comprise any type of mineral lubricating base oil, synthetic or natural, animal or vegetable, known to those skilled in the art.

The base oils used in the lubricant compositions according to the invention may be oils of mineral or synthetic origin belonging to groups I to V according to the classes defined in the API classification (or their equivalents according to the ATIEL classification) (Table A) or their mixtures.

TABLE A Saturated Sulfur Viscosity index content content (VI) Group I  <90% >0.03% 80 ≤ VI < 120 Mineral oils Group II ≥90% ≤0.03% 80 ≤ VI < 120 Hydrocracked oils Group III ≥90% ≤0.03% ≥120 Hydrocracked or hydro-isomerized oils Group IV Poyalphaolefines (PAO) Group V Esters, PAG and other bases not included in groups I to IV

The mineral base oils according to the invention include all types of base oils obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, desalphating, solvent dewaxing, hydrotreatment, hydrocracking, hydroisomerization and hydrofinition.

Mixtures of synthetic and mineral oils may also be used.

There is generally no limitation on the use of different lubricating bases to make the lubricant compositions according to the invention, except that they must have properties, in particular viscosity, viscosity index, sulfur content, oxidation resistance, which are suitable for use for engines or for vehicle transmissions.

The base oils of the lubricant compositions according to the invention may also be chosen from synthetic oils, such as certain carboxylic acid esters and alcohols, and from polyalphaolefins. The polyalphaolefins used as base oils are, for example, obtained from monomers comprising from 4 to 32 carbon atoms, for example from octene or decene, and whose viscosity at 100° C. is between 1.5 and 15 mm².s⁻¹ according to ASTM D445. Their average molecular weight is generally between 250 and 3000 according to ASTM D5296.

Preferably, the base oils of the present invention are chosen from the above base oils whose aromatic content is between 0 and 45%, preferably between 0 and 30%. The aromatic content of the oils is measured according to the Burdett UV method. Without wishing to be bound by any theory, the aromaticity of the base oil is a characteristic that makes it possible to optimize the operation of the polymer as a function of temperature. The choice of a low aromatic oil allows an optimum at a higher temperature.

Advantageously, the lubricant composition according to the invention comprises at least 50% by weight of base oils relative to the total weight of the composition. More advantageously, the lubricant composition according to the invention comprises at least 60% by weight, or even at least 70% by weight, of base oils relative to the total weight of the composition.

More particularly advantageously, the lubricant composition according to the invention comprises from 60 to 99.5% by weight of base oils, preferably from 70 to 99.5% by weight of base oils, relative to the total weight of the composition.

Many additional additives may be used for this lubricant composition according to the invention.

The additional additives that are preferred for the lubricant composition according to the invention are chosen from among detergent additives, anti-wear additives different from the phospho-sulfur additives, friction modifying additives different from the organomolybdenum compounds, extreme pressure additives, dispersants, pour point improvers, defoamers, thickeners and mixtures thereof.

Amine phosphates are anti-wear additives that may be used in the lubricant composition according to the invention. However, the phosphorus provided by these additives may act as a poison for the catalytic systems of automobiles because these additives are ash generators. These effects may be minimized by partially substituting amine phosphates with non-phosphorus additives, such as, for example, polysulfides, especially sulfur-containing olefins.

Advantageously, the lubricant composition according to the invention may comprise from 0.01 to 6% by weight, preferably from 0.05 to 4% by weight, more preferably from 0.1 to 2% by weight relative to the total weight of lubricant composition, anti-wear additives and extreme pressure additives.

Advantageously, the lubricant composition according to the invention may comprise at least one additional friction modifier additive different from the organomolybdenum compounds. The additional friction modifier additive may be selected from a compound providing metal elements and an ash-free compound. Among the compounds providing metal elements, mention may be made of transition metal complexes such as Sb, Sn, Fe, Cu, Zn, the ligands of which may be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus. The ash-free friction modifier additives are generally of organic origin and may be selected from monoesters of fatty acids and polyols, fatty epoxides, borate fatty epoxides; or glycerol esters of fatty acid. According to the invention, the fatty compounds comprise at least one hydrocarbon group comprising from 10 to 24 carbon atoms.

Advantageously, the lubricant composition according to the invention may comprise at least one antioxidant additive.

The antioxidant additive generally serves to retard the degradation of the lubricant composition in service. This degradation may, in particular, result in the formation of deposits, the presence of sludge or an increase in the viscosity of the lubricant composition.

Antioxidant additives act, in particular, as radical inhibitors or destroyers of hydroperoxides. Among the antioxidant additives commonly used, mention may be made of antioxidant additives of the phenolic type, antioxidant additives of the amine type, antioxidant phosphosulfur additives. Some of these antioxidant additives, for example phosphosulfur antioxidant additives, may be ash generators. Phenolic antioxidant additives may be ash-free or may be in the form of neutral or basic metal salts. The antioxidant additives may be chosen, in particular, from sterically-hindered phenols, sterically-hindered phenol esters and sterically-hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted with at least one C₁-C₁₂ alkyl group, N,N′-dialkyl-aryl diamines, and mixtures thereof.

Preferably, according to the invention, the sterically-hindered phenols are chosen from compounds comprising a phenol group in which at least one vicinal carbon of the carbon bearing the alcohol function is substituted by at least one C₁-C₁₀ alkyl group, preferably a C₁-C₆ alkyl group, preferably a C₄ alkyl group, preferably by the ter-butyl group.

Amino compounds are another class of antioxidant additives that may be used optionally in combination with phenolic antioxidant additives. Examples of amine compounds are aromatic amines, for example aromatic amines of formula NR⁷R⁸R⁹ in which R⁷ represents an optionally substituted aliphatic or aromatic group, R⁸ represents an optionally substituted aromatic group, R⁹ represents a hydrogen atom, an alkyl group, an aryl group or a group of formula R¹⁰S(O)_(z)R¹¹ in which R¹⁰ represents an alkylene group or an alkenylene group, R¹¹ represents an alkyl group, an alkenyl group or an aryl group and z represents 0, 1 or 2.

Sulfurized alkyl phenols or their alkali and alkaline earth metal salts may also be used as antioxidant additives.

Another class of antioxidant additives is copper compounds, for example copper thio- or dithio-phosphates, copper and carboxylic acid salts, dithiocarbamates, sulphonates, phenates, copper acetylacetonates. Copper salts I and II, succinic acid or anhydride salts may also be used.

The lubricant composition according to the invention may contain all types of antioxidant additives known to those skilled in the art.

Advantageously, the lubricant composition comprises at least one ash-free antioxidant additive.

Also advantageously, the lubricant composition according to the invention comprises from 0.5 to 2% by weight relative to the total weight of the composition, of at least one antioxidant additive.

The lubricant composition according to the invention may also comprise at least one detergent additive.

The detergent additives generally make it possible to reduce the formation of deposits on the surface of the metal parts by dissolving the secondary oxidation and combustion products.

The detergent additives that may be used in the lubricant composition according to the invention are generally known to those skilled in the art. The detergent additives may be anionic compounds comprising a long lipophilic hydrocarbon chain and a hydrophilic head. The associated cation may be a metal cation of an alkali metal or alkaline earth metal.

The detergent additives are preferably chosen from among the alkali metal or alkaline earth metal salts of carboxylic acids, the sulphonates, the salicylates, the naphthenates and the phenate salts. The alkali and alkaline earth metals are preferably calcium, magnesium, sodium or barium.

These metal salts generally comprise the metal in stoichiometric amount or in excess, therefore in an amount greater than the stoichiometric amount. It then relates to overbased detergent additives; the excess metal bringing the overbased character to the detergent additive is then generally in the form of a metal salt insoluble in oil, for example a carbonate, a hydroxide, an oxalate, an acetate, a glutamate, preferably a carbonate.

Advantageously, the lubricant composition according to the invention may comprise from 2 to 4% by weight of detergent additive relative to the total weight of the lubricant composition.

Also advantageously, the lubricant composition according to the invention may also comprise at least one pour point depressant additive.

By slowing the formation of paraffin crystals, pour point depressant additives generally improve the cold behavior of the lubricant composition according to the invention.

As examples of pour point depressant additives, mention may be made of alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes and alkylated polystyrenes.

Advantageously, the lubricant composition according to the invention may also comprise at least one dispersing agent.

The dispersing agent may be chosen from among Mannich bases, succinimides and their derivatives.

Also advantageously, the lubricant composition according to the invention may comprise from 0.2 to 10% by weight of dispersing agent relative to the total weight of the lubricant composition.

The lubricant composition of the present invention may also comprise at least one additional polymer improving the viscosity index. Examples of additional polymers improving the viscosity index, may be mentioned polymeric esters, homopolymers or copolymers, hydrogenated or non-hydrogenated, styrene, butadiene and isoprene, polymethacrylates (PMA).

The present invention also relates to the use of the lubricant composition as defined above for engine lubrication, in particular an internal combustion engine, for example a vehicle engine.

Advantageously, the lubricant composition according to the invention makes it possible to reduce friction, in particular between two parts of an engine, in particular an internal combustion engine, for example a vehicle engine.

Thus, the invention relates to the use of the lubricant composition according to the invention for reducing wear on the parts of an engine, for example a vehicle engine.

The present application also relates to a method of lubricating mechanical parts, in particular in an engine, in particular an internal combustion engine, comprising at least one step of contacting at least one part with a lubricant composition according to the invention.

The present invention will now be described by way of non-limiting examples.

FIG. 1 represents the X-ray photoelectron spectrum (the spectrum gives the binding energies (eV)) produced on the lubricant films (from compositions according to the invention) obtained at the end of tribological tests on steel/steel parts.

EXAMPLE 1 Lubricant Compositions

The compositions of Table 1 (CL: lubricant composition according to the invention, CC: comparative composition) were prepared by mixing the TiO₂ particles and the organomolybdenum compound in a base oil at 60° C. to obtain a good dispersion of the particles in the composition.

TABLE 1 Lubricant compositions according to the invention and comparatives Lubricant composition according to CL1 (% CL2 (% CL3 (% CL4 (% CC1 (% CC2 (% CC3 (% the invention in weight) in weight) in weight) in weight) in weight) in weight) in weight) Anatase TiO₂ 0.5 0.1 0.5 0.5 particles Rutile TiO₂ 0.5 0.5 particles MoDTC 0.5 0.5 0.1 0.5 0.5 Group III 99 99 99.8 98 99.5 99.5 99.5 base oil ZDDP 1

EXAMPLE 2 Results of Tribological Tests

2.1 Tests on Alternative Tribometer Ball-Plane

Measurements of the coefficient of friction and the wear of the balls in micrometers in diameter of the ball impression (Table 2) were carried out on an alternating ball-plane tribometer. These tests were performed by changing for each test the mechanical parts of a flat surface. The parts used are as follows:

-   -   the balls have a diameter of 5 mm and are mechanical parts made         of reference steel AISI52100 having a surface roughness (Ra) of         50 nm;     -   the plane is a flat mechanical part chosen from:         -   PM1: a reference steel AISI52100; or         -   PM2: made of APS steel plasma treated steel.

where

PM1 has a surface roughness (Ra) of 50 nm.

PM2 has a surface roughness (Ra) of between 170 and 200 nm.

The conditions of the alternative tribometer ball-plane test are:

-   -   Temperature: 100° C.     -   Frequency: 5 Hz     -   Maximum contact pressure: 700 MPa     -   Trace length: 5 mm     -   Duration: 1h     -   Volume of lubricant composition: 2-3 mL

The coefficients of friction and the wear of the balls in μm in diameter of the impression of the ball indicated in Table 2 were measured following the contact of a smooth surface ball (Ra=50 nm) with respectively a flat part PM1 smooth surface (Ra=50 nm) or a flat part PM2 rough surface (Ra=170-200 nm), said parts also being in contact with a composition according to the invention or a comparative composition.

TABLE 2 Coefficients of friction and wear of the ball measured on an alternating ball-plane tribometer during the contacts between the ball and the parts PM1 and PM respectively, the parts also being in contact with a composition according to the invention or a comparative composition Coefficient of friction/ wear of the Base oil balls in mg alone CL1 CL3 CC1 CC3 Ball/PM1 0.16/370 0.039/184 nd 0.12/444 0.05/215 Ball/PM2 0.12/300 0.036/275 0.031/230 0.13/290 0.05/275 Nd: not determined

These results demonstrate that:

-   -   The coefficient of friction between ball/PM1 is decreased when         the lubricant composition comprises an organomolybdenum compound         and titanium, whatever the content of organomolybdenum compound         and titanium. This coefficient of friction being measured         between two pieces of smooth surface, makes it possible to         determine the formation of MoS₂ from the organomolybdenum         compound. Nevertheless, in the case of the PM1 parts having         smooth surfaces, the friction contacts are decreased relative to         the friction contacts between a smooth surface piece and a rough         surface piece such as PM2.     -   The wear of the balls is lower when there is a combination         between a composition according to the invention (CL1 or CL3)         and the base oil in comparison with the base oil alone or with         the comparative compositions (CC1, CC3 or CC4), for both types         of surfaces (smooth or rough).     -   When the concentration of organomolybdenum compound is decreased         (composition CL3), there is a more significant decrease in the         coefficient of friction and the wear of the balls both with         respect to the oil alone and compared to the comparative         compositions.     -   The coefficient of friction between ball/PM2 (rough surface) is         decreased when the lubricant composition comprises an         organomolybdenum compound and titanium, regardless of the         content of organomolybdenum compound and titanium.     -   That there is a synergy between the molybdenum and the titanium         particles within the lubricant composition that significantly         reduces the coefficient of friction and therefore to limit         friction between the parts.

EXAMPLE 3 X-Ray Spectra

X-ray photoelectron spectroscopies were performed on the lubricant films (from compositions CL1 and CC3) obtained at the end of the tribological ball/PM1 tests.

FIG. 1 shows the top spectrum of composition CC3 and bottom spectrum of composition CL1. This figure shows that in the absence of TiO₂ particles (CC3) decomposition of MoDTC is not complete and gives rise to the formation of MoS₂ and MoOxSy Mo oxysulfide in large quantities. On the contrary the presence of TiO₂ allows a better decomposition of the MoDTC and the formation of pure MoS₂. 

1. Lubricant composition comprising: at least one base oil; from 0.05 to 1.5% by weight of an organomolybdenum compound; and from 0.005 to 1% by weight of TiO₂ particles.
 2. Composition according to claim 1, wherein the particles have an average size of between 10 nm and 1 μm.
 3. Composition according to claim 1, comprising from 0.08 to 1% by weight of organomolybdenum compound relative to the total weight of said composition.
 4. Composition according to claim 1, comprising from 0.01 to 0.8% by weight of titanium particles relative to the total weight of said composition.
 5. Composition according to claim 1, in which the organomolybdenum compound is chosen from organic complexes of molybdenum comprising at least one molybdenum (Mo) chemical element and at least one ligand chosen from carboxylate and ester, amide, dithiophosphate or dithiocarbamate ligands.
 6. Composition according to claim 1, wherein the organomolybdenum compound is chosen from organic complexes of molybdenum with amide ligands obtained by reaction: (i) a mono, di or tri glyceride fatty substance, or fatty acid, (ii) an amine source of formula (A):

in which: X¹ represents an oxygen atom or a nitrogen atom, X² represents an oxygen atom or a nitrogen atom, n or m represents 1 when X¹ or X² respectively represents an oxygen atom, n or m represents 2 when X1 or X2 respectively represents a nitrogen atom, (iii) and a molybdenum source selected from molybdenum trioxide or molybdates, preferably ammonium molybdate.
 7. Composition according to claim 1, wherein the organomolybdenum compound comprises at least one organic complex of molybdenum of formula (III) or (IV), alone or as a mixture:

in which: X¹ represents an oxygen atom or a nitrogen atom; X² represents an oxygen atom or a nitrogen atom; n represents 1 when X¹ represents an oxygen atom and m represents 1 when X² represents an oxygen atom; n represents 2 when X¹ represents a nitrogen atom and m represents 2 when X² represents a nitrogen atom; R₁ represents a linear or branched, saturated or unsaturated alkyl group comprising from 4 to 36 carbon atoms, preferably from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon atoms;

in which: X¹ represents an oxygen atom or a nitrogen atom; X² represents an oxygen atom or a nitrogen atom; n represents 1 when X¹ represents an oxygen atom and m represents 1 when X² represents an oxygen atom; n represents 2 when X¹ represents a nitrogen atom and m represents 2 when X² represents a nitrogen atom; R₁ represents a linear or branched, saturated or unsaturated alkyl group comprising from 4 to 36 carbon atoms, preferably from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon atoms; R₂ represents a linear or branched, saturated or unsaturated alkyl group comprising from 4 to 36 carbon atoms, preferably from 4 to 20 carbon atoms, advantageously from 6 to 18 carbon atoms.
 8. Composition according to claim 1, wherein the organomolybdenum compound is selected from organic complexes of molybdenum with dithiophosphate ligands or organic complexes of molybdenum with dithiocarbamate ligands.
 9. Composition according to claim 1, wherein the organomolybdenum compound is a molybdenum dithiocarbamate compound (MoDTC). 10-12. (canceled)
 13. Method for lubricating mechanical parts, in particular in an engine, in particular an internal combustion engine, comprising at least one step of contacting at least one part with a lubricant composition according to claim
 1. 14. Method for reducing friction in an engine, preferably internal combustion engine, for example vehicle engine, comprising at least one step of contacting at least one part of the engine with a lubricant composition according to claim
 1. 15. Method for reducing wear in an engine, preferably internal combustion engine for example vehicle engine, comprising at least one step of contacting at least one part of the engine with a lubricant composition according to claim
 1. 